Wild birds 'partly' to blame for bird flu spread

Wild birds have helped transmit the deadly H5N1 bird flu across Eurasia, a meeting of 300 scientists at the UN Food and Agriculture Organisation (FAO) concluded on Wednesday. But killing them to prevent further spread of the disease is not the answer, they warn.

In fact, Marc Choisy and Pejman Rohani at the University of Georgia at Athens in the US have shown that killing wild animals with a disease like flu could actually lead to more infected animals, not fewer.

This is due to a classic principle of ecology, called compensation. Many wild species produce more offspring than can survive. Hunting removes animals which would otherwise have competed with these excess young, especially as hunters often target bigger, older beasts. So in a hunted population, more young usually survive which compensates – or sometimes even over-compensates – for the loss due to hunting.

But when a disease causes lifelong immunity in its host, most of the older animals in a population have survived it and are therefore immune, leaving only the young susceptible. Flu is such a disease – you catch it repeatedly only because the virus itself changes to evade your immunity. And among wild aquatic birds, flu is most prevalent at the end of the breeding season, when there are the most susceptible young birds about.

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Increased carriers

In a detailed mathematical model, Choisy and Rohani show that by increasing the proportion of young in a population through hunting, the total number of cases of a disease like flu actually increases.

When the disease kills these animals, this will add increased deaths from illness to deaths from hunting, possibly threatening the species. But where the animals do not die from the disease – as is the case with some wild birds infected with H5N1, such as dabbling ducks– this can actually increase the number of carriers that can pass it to other animals.

The same principle, the authors note, might explain why killing badgers and foxes fails to reduce the incidence of bovine tuberculosis and rabies, respectively, which these animals spread.

Long distance

The meeting in Rome was called by the FAO and the World Animal Health Organisation in order to clarify the role wild birds play in spreading H5N1, following considerable resistance to the idea within the ornithological community.

It concluded that while poultry has dominated the spread of the disease, wild birds have also played a role, particularly in transmitting the H5N1 virus long distances across Eurasia during migration.

The FAO’s chief vet, Juan Lubroth, says “we don’t need prime ministers to come out and say, ‘we’ll cut off the tops of trees or drain the wetlands’ [to kill the carriers]”. Instead, scientists at the meeting called for increased research to see which species carry the virus, whether it can persist in wild bird populations, and to where the birds migrate.

Journal reference&colon; Proceedings of the Royal Society (DOI&colon; 10.1098/rspb.2006.3554)